1. Field of the Invention
This invention relates to the collection and processing of drill cuttings separated from a drilling rig's solids control system and more particular to the processing and injections of such cuttings into fractures in the earth formation adjacent the well being drilled via the annulus between a well casing and well bore or into other such cuttings disposal scenarios.
2. General Background
In the oil and gas drilling industry the processing of drill cuttings and their disposal has been a logistics and environmental problem for a number of years. Various systems have been developed for handling and processing the cuttings for disposal and reclamation. Such systems include returning the cuttings via injection under high pressure back into the earth formation in a manner such as that described in U.S. Pat. Nos. 4,942,929, 5,129,469 and 5,109,933, and the treatment of drill cuttings as disclosed by U.S. Pat. Nos. 4,595,422 5,129,468, 5,361,998 and 5,303,786. However, in practice, the injection process is not as simple as it may seem. The preparation of the cuttings into a homogeneous mix which is acceptable to high pressure pumps used in pumping material down a well is essential. Transforming the cuttings into a pumpable slurry is complicated by variable drill rates producing large volumes of cuttings at times thereby creating surges in drill waste materials, the need to pump the slurry at high pressures into the earth and/or formation fractures hundreds if not thousands of feet below the surface. Complications also arise due to the need for constant velocity and high horsepower while pumping. On offshore platforms space is at a premium. Therefore, cuttings treatment units must be compact and as light in weight as possible. Solids control equipment is most often placed in hazardous areas, near the well bore, where large horsepower internal combustion engines are not permitted due to the possibility of high gas concentration. Therefore, any additional equipment used for processing solids must meet stringent explosion proof requirements for such areas of the rig.
Heretofore, cuttings injection has not gained wide acceptance in offshore drilling operations such as may be found in the North Sea, primarily due to the problems discussed above and the inefficiency and ineffectiveness of the cuttings preparation and injection processes.
Although, other cuttings processing system have been developed for preparing drill cutting for disposal and some have been tried in an attempt to inject such processed drill cuttings into a well bore, as is disclosed by U.S. Pat. Nos. 4,942,929, 5,129,469, and 5,109,933 and 5,431,236. However, none combine, individually or collectively all of the advanced features, required for problem-free cuttings injection, disclosed herein by the instant invention.
The problems associated with cuttings injection are numerous as expressed by Warren in U.S. Pat. No. 5,431,236. Starting with processing of the cuttings for injection, we find that the particles are not uniform in size and density making the slurification process very complicated. The cuttings mixture often plugs circulating pumps, the abrasiveness of the cuttings also abrade the pump impellers causing cracking, some attempts have been made to use the circulating pumps for grinding the injection particles by purposely causing pump cavitaion, thereby shortening pump life, hard cakes build up in tanks creating circulation problems and circulation pumps cavitate unexpectedly due to irregular particle size. Therefore, it is known that a uniform particle size of less than 100 micron must be maintained for proper formation injection at the well site. Maintaining such consistency with hard and soft materials is very difficult. The use of shear guns to reduce particle size as taught by Warren does not insure consistency and requires continuous recalibration thereby reducing the volume capacity of the processor. Warren also teaches that sand should be separated through the use of hydrocyclones which further reduces throughput volume.
Next we find that since no two earth formations are alike it is very difficult to prevent plugging of the formation fractures in the well bore especially when there are long delays in placement of the injection slurry in the formation. Plugging of the formation fractures often occurs as a direct result of large particle size, often in the range of 300 micron or greater, combined with high pressure high volume applications. Plugging of the well formation results in extensive well drilling downtime which is very expensive.
Cuttings injection failures have occurred primarily due to the inability to, handle large volumes of cuttings surges, fine tune the injection process by providing particle size control, uniform slurry density and to provide volume and pressure control over the injection process. Further, attempts to inject cutting slurries into the earth have met with failure as a result of the inability to manually control all facets of the process and injection operation. As a result of such failures most offshore drilling operators in the North Sea have ban the practice and have resorted to using expensive synthetic drill fluids.
It is to this end that the present invention has been developed, the proprietary know-how of which has been maintained until disclosed herein thereby, disclosing a unique efficient system and method for injecting drill cuttings into an offshore oil and gas well in a drilling environment requiring compactness, relatively light weight, low maintenance, full automation and operability in hazardous potentially explosive environments.